The mitral valve acts as a check valve to prevent flow of oxygenated blood back into the left atrium during systole, i.e., when the left ventricle contracts. This allows oxygenated blood to pump into the aorta through the aortic valve. Regurgitation of the mitral valve can significantly decrease the pumping efficiency of the heart and thereby increase the risk of severe, progressive heart failure. Mitral valve regurgitation can be characterized by retrograde flow from the left ventricle of a heart through an incompetent mitral valve into the left atrium. Mitral valve regurgitation can result from a number of mechanical defects. For example, leaflets, chordae tendineae coupled to the leaflets, and/or the papillary muscles of the mitral valve may be damaged or otherwise dysfunctional. In at least some instances, the mitral valve annulus itself may be damaged, dilated, or weakened such that the mitral valve does not close adequately during systole.
One mechanism for treating mitral valve regurgitation is mitral valve replacement. Percutaneous mitral valve replacement is significantly more challenging than aortic valve replacement because the native mitral valve and surrounding structures pose unique anatomical obstacles. Unlike the relatively symmetric and uniform native aortic valve, the mitral valve annulus has a non-circular, D-shape or kidney-like shape, with a non-planar saddle-like geometry. Such complexity makes it difficult to design a mitral valve prosthesis that conforms adequately to the mitral annulus to prevent leakage and backflow. For example, gaps between the prosthesis and the native tissue allow backflow of blood through the gaps from the left ventricle to the left atrium. As a result, cylindrical valve prostheses may leave gaps in commissural regions of the native valve that potentially result in perivalvular leaks in those regions.
In addition to its irregular, unpredictable shape, which changes size over the course of each heartbeat, the mitral valve annulus also lacks a significant amount of radial support from surrounding tissue. The aortic valve, for example, is completely surrounded by fibro-elastic tissue that provides the native structural support to anchor a prosthetic valve. The inner wall of the native mitral valve, however, is bound by a thin vessel wall separating the mitral valve annulus from the inferior portion of the aortic outflow tract. As a result, significant radial forces on the mitral annulus, such as those exerted by expanding stent prostheses, could cause collapse of the inferior portion of the aortic tract. Further, since the chordae tendineae extend from the papillary muscles to the underside of the leaflets, deploying a valve prosthesis on the ventricular side of the native mitral annulus is challenging. Thus, prosthetic mitral valves must accommodate the difficult anatomy of the mitral valve and surrounding structures.